EXHIBIT 10.21 PURCHASE AGREEMENT

QuickLinks -- Click here to rapidly navigate through this document

Exhibit 10.21

$200,000,000 9 ¹ / 2 % Senior Notes due 2009
$260,000,000 11% Senior Notes due 2012
LABRANCHE & CO INC.
PURCHASE AGREEMENT

May 4, 2004

Credit Suisse First Boston LLC,
Eleven Madison Avenue,
New York, N.Y. 10010-3629

Dear Sirs:

        1.     Introductory .    LaBranche & Co Inc., a Delaware corporation (the " Company "), proposes, subject to the terms and conditions stated herein, to issue and sell to Credit Suisse First Boston LLC (the " Initial Purchaser ") (i) $200,000,000 principal amount of its 9 ¹ / 2 % Senior Notes due 2009 (the " 2009 Notes ") and (ii) $260,000,000 principal amount of its 11% Senior Notes due 2012 (the " 2012 Notes " and, together with the 2009 Notes, the " Offered Securities ") to be issued under an indenture dated as of May 18, 2004 (the " Indenture "), between the Company and U.S. Bank National Association, as trustee (the " Trustee "), on a private placement basis pursuant to an exemption under Section 4(2) of the United States Securities Act of 1933 (the " Securities Act "), and hereby agrees with the Initial Purchaser as follows.

        The holders of the Offered Securities will be entitled to the benefits of a Registration Rights Agreement of even date herewith between the Company and the Initial Purchaser (the " Registration Rights Agreement "), pursuant to which the Company agrees to file (i) a registration statement (" Exchange Offer Registration Statement ") with the Securities and Exchange Commission (the " Commission ") with respect to a proposed offer (the " Registered Exchange Offer ") to the holders of the Offered Securities, to issue and deliver to such holders, in exchange for the Offered Securities, a like aggregate principal amount of debt securities (the " Exchange Securities ") of the Company issued under the Indenture and identical in all material respects to the Offered Securities (except for the transfer restrictions relating to the Offered Securities and certain other provisions discussed in Section 6 of the Registration Rights Agreement) that would be registered under the Securities Act, and (ii) a shelf registration statement pursuant to Rule 415 under the Securities Act under certain circumstances specified in the Registration Rights Agreement.

        2.     Representations and Warranties of the Company .    The Company represents and warrants to, and agrees with, the Initial Purchaser that:

        (a)    A preliminary offering circular and an offering circular relating to the Offered Securities have been prepared by the Company. Such preliminary offering circular (the " Preliminary Offering Circular ") and offering circular (the " Offering Circular "), as supplemented as of the date of this Agreement, together with any other document approved by the Company for use in connection with the contemplated resale of the Offered Securities, are hereinafter collectively referred to as the " Offering Document ". The Preliminary Offering Circular as of its date did not, and the Offering Circular on the date of this Agreement does not, include any untrue statement of a material fact or omit to state any material fact required to be stated therein or necessary in order to make the statements therein, in the light of the circumstances under which they were made, not misleading. The preceding sentence does not apply to statements in or omissions from the Offering Document based upon written information furnished to the Company by the Initial Purchaser specifically for use therein, it being understood and agreed that the only such information is that described as such in Section 7(b) hereof. Except as disclosed in the Offering Document, on the

date of this Agreement, the Company's Annual Report on Form 10-K for the fiscal year ended December 31, 2003 (the " 2003 Form 10-K ") filed with the SEC, and all subsequent reports to the 2003 Form 10-K (collectively, the " Exchange Act Reports ") which have been filed by the Company with the Commission or sent to stockholders pursuant to the Securities Exchange Act of 1934 (the " Exchange Act ") do not include any untrue statement of a material fact or omit to state any material fact necessary to make the statements therein, in the light of the circumstances under which they were made, not misleading. Such documents, when they were filed with the Commission, conformed in all material respects to the requirements of the Exchange Act and the rules and regulations of the Commission thereunder. No stop order or decree preventing the use of the Offering Document, or any amendment or supplement thereto, or any order asserting that any of the transactions contemplated by this Agreement are subject to the registration requirements of the Securities Act, has been issued and no proceeding for any such purpose has been commenced or is pending or, to the knowledge of the Company, is threatened.

        (b)    The Company has been duly incorporated, is validly existing as a corporation in good standing under the laws of its jurisdiction of incorporation and has full corporate power and authority to carry on its business as described in the Offering Document and to own, lease and operate its properties, and is duly qualified and is in good standing as a foreign corporation authorized to do business in each jurisdiction in which the nature of its business or its ownership or leasing of property requires such qualification, except where the failure to be so qualified or in good standing in a foreign jurisdiction would not have a material adverse effect on the business, condition (financial or other), properties, or results of operations of the Company and its subsidiaries set forth on Exhibit A hereto (each, a " Subsidiary " and collectively, the " Subsidiaries "), taken as a whole, or draw into question the validity of this Agreement or the Indenture or the Registration Rights Agreement (a " Material Adverse Effect "). As of the date hereof, the Subsidiaries constitute all of the Company's subsidiaries.

        (c)    Each Subsidiary has been duly incorporated or organized, as the case may be, and is an existing corporation or limited liability company, as the case may be, in good standing under the laws of the jurisdiction of its incorporation or organization, as the case may be, with power and authority (corporate and other) to own, lease and operate its properties and conduct its business as described in the Offering Document; and each Subsidiary is duly qualified to do business as a foreign corporation and is in good standing in all other jurisdictions in which its ownership or leasing of property or the conduct of its business requires such qualification, except to the extent that a failure to be so qualified or in good standing in a foreign jurisdiction would not have a Material Adverse Effect. All of the issued and outstanding capital stock or membership interests, as the case may be, of each Subsidiary have been duly authorized and validly issued and are fully paid and nonassessable, and are owned by the Company, directly or through the Subsidiaries, free and clear of any security interest, claim, lien, encumbrance or adverse interest of any nature (each, a " Lien ").

        (d)    At December 31, 2003, the Company had an authorized capitalization as set forth in the Offering Document, and all currently outstanding shares of capital stock of the Company have been duly authorized and validly issued and are fully paid, nonassessable and not subject to any preemptive or similar rights.

        (e)    The Indenture has been duly authorized by the Company; the Offered Securities have been duly authorized by the Company; and when the Offered Securities are delivered and paid for pursuant to this Agreement on the Closing Date (as defined below), the Indenture will have been duly executed and delivered by the Company, and such Offered Securities will have been duly executed, authenticated, issued and delivered by the Company, will conform to the description thereof contained in the Offering Document and be entitled to the benefits of the Indenture. The Indenture and such Offered Securities will constitute valid and legally binding obligations of the

2

Company, enforceable in accordance with their terms, subject to bankruptcy, insolvency, fraudulent transfer, reorganization, moratorium and similar laws of general applicability relating to or affecting creditors' rights and to general equity principles. On the Closing Date, the Indenture will conform in all material respects to the description thereof in the Offering Document and to the requirements of the Trust Indenture Act of 1939, as amended (the " Trust Indenture Act "), and the rules and regulations of the Commission applicable to an indenture which is qualified thereunder.

        (f)    The Registration Rights Agreement has been duly authorized, executed and delivered by the Company and is a valid and binding agreement of the Company, enforceable against the Company in accordance with its terms, subject to bankruptcy, insolvency, fraudulent transfer, reorganization, moratorium and similar laws of general applicability relating to or affecting creditors' rights and to general equity principles. On the Closing Date, the Registration Rights Agreement will conform in all material respects to the description thereof in the Offering Document.

        (g)    On the Closing Date, the Exchange Securities will have been duly authorized by the Company and when the Exchange Securities are issued, executed and authenticated in accordance with the terms of the Exchange Offer and the Indenture, the Exchange Securities will be entitled to the benefits of the Indenture and will be valid and legally binding obligations of the Company, enforceable in accordance with their terms, subject to bankruptcy, insolvency, fraudulent transfer, reorganization, moratorium and similar laws of general applicability relating to or affecting creditors' rights and to general equity principles.

        (h)    This Agreement has been duly authorized, executed and delivered by the Company.

        (i)    Neither the Company nor any of the Subsidiaries is (i) in violation of its charter, by-laws or other organizational documents or (ii) in default in the performance of any obligation, agreement, covenant or condition contained in any indenture, loan agreement, mortgage, lease or other agreement or instrument to which the Company or any of the Subsidiaries is a party or by which the Company or any of the Subsidiaries or their respective property is bound, except, in the case of clause (ii), for any such violation or default that individually or in the aggregate could not reasonably be expected to have a Material Adverse Effect.

        (j)    The execution, delivery and performance of this Agreement, the Indenture and the Registration Rights Agreement by the Company, the compliance by the Company with all provisions hereof and thereof and the consummation of the transactions contemplated hereby and thereby will not (i) require any consent, approval, authorization or other order of, or filing, or qualification with, any court or governmental or self regulatory body or agency (except such as may be required under federal securities laws, the securities or Blue Sky laws of the various states, the rules and regulations of the various self-regulatory organizations or the Trust Indenture Act), (ii) conflict with or constitute a breach of any of the terms or provisions of, or a default under, (a) the charter, bylaws or other organizational documents of the Company or any of the Subsidiaries or (b) any indenture, loan agreement, mortgage, lease or other agreement or instrument to which the Company or any of the Subsidiaries is a party or by which the Company or any of the Subsidiaries or any of its or their property is bound, except, in the case of clause (b), any such conflict, breach or default that individually or in the aggregate could not reasonably be expected to have a Material Adverse Effect, (iii) violate or conflict with any applicable statute, law, ordinance, administrative, governmental or self-regulatory organizational rule or regulation, or judgment, order or decree of any court or any governmental body or agency having jurisdiction over the Company, any of the Subsidiaries or any of its or their property, except any such violation or conflict that individually or in the aggregate could not reasonably be expected to have a Material Adverse Effect, (iv) result in the imposition or creation of (or the obligation to create or impose) a material Lien under, any agreement or instrument to which the Company or any of the

3

Subsidiaries is a party or by which the Company or any of the Subsidiaries or any of its or their property is bound, or (v) result in the termination, suspension or revocation of any Authorization (as defined below) of the Company or any of the Subsidiaries or result in any other impairment of the rights of the holder of any such Authorization.

        (k)    Except as disclosed in the Offering Document, the Company and the Subsidiaries have good and marketable title to all real properties and all other properties and assets owned by them, in each case free from Liens that would materially affect the value thereof or materially interfere with the use made or to be made thereof by them; and except as disclosed in the Offering Document, the Company and the Subsidiaries hold any leased real or personal property under valid and enforceable leases with no exceptions that would materially interfere with the use made or to be made thereof by them.

        (l)    The Company and the Subsidiaries own, possess or can acquire on reasonable terms, adequate trademarks, trade names and other rights to inventions, know-how, patents, copyrights, confidential information and other intellectual property (collectively, " Intellectual Property Rights ") necessary to conduct the business now operated by them, or presently employed by them, and have not received any notice of infringement of or conflict with asserted rights of others with respect to any Intellectual Property Rights that, if determined adversely to the Company or any of the Subsidiaries, individually or in the aggregate could reasonably be expected to have a Material Adverse Effect.

        (m)    Except as disclosed in the Offering Document or incorporated by reference into the Offering Document, there are no legal or governmental proceedings pending or, to the knowledge of the Company, threatened to which the Company or any of the Subsidiaries is or could be a party or to which any of its or their property is or could be subject, that individually or in the aggregate, could reasonably be expected to have a Material Adverse Effect. There are no such proceedings that materially adversely affect the issuance of the Offered Securities or any of the transactions contemplated by this Agreement, the Indenture and the Registration Rights Agreement. Except as disclosed in the Offering Document or incorporated by reference into the Offering Document, neither the Company nor any of the Subsidiaries is involved in any strike, job action or labor dispute with any group of its employees that individually or in the aggregate could reasonably be expected to have a Material Adverse Effect, and, to the knowledge of the Company, no such action or dispute is threatened.

        (n)    (i) Arthur Andersen LLP, which has certified certain financial statements of the Company and its consolidated subsidiaries and delivered its report with respect to certain of the audited consolidated financial statements and schedule incorporated by reference in the Offering Circular, were, at the time of delivery of such report, independent public accountants with respect to the Company within the meaning of the Securities Act and the Exchange Act and the then-applicable published rules and regulations thereunder.

        (ii)    KPMG LLP, which has certified certain financial statements of the Company and its consolidated subsidiaries and delivered their report with respect to certain of the audited consolidated financial statements incorporated by reference in the Offering Circular, are independent public accountants with respect to the Company within the meaning of the Securities Act and the Exchange Act and the applicable published rules and regulations thereunder.

        (o)    The financial statements and financial statement schedule incorporated by reference in the Offering Document present fairly the financial position of the Company and its consolidated subsidiaries as of the dates shown and their results of operations and cash flows for the periods shown, and such financial statements have been prepared in conformity with generally accepted accounting principles in the United States applied on a consistent basis. The selected financial data

4

set forth under the captions "Summary—Summary Historical Consolidated Financial Data" and "Selected Historical Consolidated Financial Data" in the Preliminary Offering Circular and the Offering Circular fairly present, on the basis stated in the Preliminary Offering Circular and the Offering Circular, the information included therein.

        (p)    The statistical and market-related data included in the Offering Document are based on or derived from sources that the Company believes to be reliable and accurate.

        (q)    The Company and each of the Subsidiaries maintain a system of internal accounting controls sufficient to provide reasonable assurance that (i) transactions are executed in accordance with management's general or specific authorizations; (ii) transactions are recorded as necessary to permit preparation of financial statements in conformity with generally accepted accounting principles and to maintain asset accountability; (iii) access to assets is permitted only in accordance with management's general or specific authorization; and (iv) the recorded accountability for assets is compared with the existing assets at reasonable intervals and appropriate action is taken with respect to any differences.

        (r)    Neither the Company nor any of the Subsidiaries has violated any foreign, federal, state or local law or regulation relating to the protection of human health and safety, the environment or hazardous or toxic substances or wastes, pollutants or contaminants (" Environmental Laws "), any provisions of the Employee Retirement Income Security Act of 1974, as amended (" ERISA "), or any provisions of the Foreign Corrupt Practices Act or the rules and regulations promulgated thereunder, except for such violations that individually or in the aggregate could not reasonably be expected to have a Material Adverse Effect.

        (s)    There are no costs or liabilities associated with Environmental Laws (including, without limitation, any capital or operating expenditures required for clean-up, closure of properties or compliance with Environmental Laws or any Authorization, any related constraints on operating activities and any potential liabilities to third parties) that individually or in the aggregate could reasonably be expected to have a Material Adverse Effect.

        (t)    Each of the Company and the Subsidiaries (i) has such concessions, permits, licenses, consents, exemptions, franchises, authorizations, orders, registrations, qualifications and other approvals (each, an " Authorization ") of, and has made all filings with and notices to, all federal, state and foreign governments, governmental or regulatory authorities and self-regulatory organizations and all courts and other tribunals, including, without limitation, under any applicable Environmental Laws and (ii) is a member in good standing of each federal, state or foreign exchange, board of trade, clearing house or association and self-regulatory or similar organization, in each case, as are necessary to own, lease, license and operate its properties and to conduct its business, except where the failure to have any such Authorization or to make any such filing or notice individually or in the aggregate could reasonably be expected to have a Material Adverse Effect. Each such Authorization is valid and in full force and effect and each of the Company and the Subsidiaries is in compliance with all the terms and conditions thereof and with the rules and regulations of the authorities and governing bodies having jurisdiction with respect thereto; and no event has occurred (including, without limitation, the receipt of any notice from any authority or governing body) which allows or, after notice or lapse of time or both, would allow, revocation, suspension or termination of any such Authorization or results or, after notice or lapse of time or both, would result in any other impairment of the rights of the holder of any such Authorization; except where such revocation, suspension, termination or impairment individually or in the aggregate could reasonably be expected to have a Material Adverse Effect.

        (u)    Each of the Company and the Subsidiaries and ERISA Affiliates (as defined below) has fulfilled its obligations, if any, under the minimum funding standards of Section 302 of ERISA and Section 412 of the Internal Revenue Code of 1986, as amended (the " Code ") and the regulations

5

and published interpretations thereunder with respect to each "plan" (as defined in Section 3(3) of ERISA) for which the Company or any of the Subsidiaries or ERISA Affiliates have or may have any liability (contingent or otherwise) and each such plan is in compliance in all material respects with the applicable provisions of ERISA and such regulations and published interpretations thereunder. Neither the Company nor any ERISA Affiliate has obtained or applied for a waiver of the minimum funding standards of Section 302 of ERISA or Section 412 of the Code with respect to such plan. Neither the Company nor any of the Subsidiaries or ERISA Affiliates has any material unpaid liability, or reasonably expects to incur any material liability, to the Pension Benefit Guaranty Corporation (other than for the payment of premiums in the ordinary course) or to any such plan under Title IV of ERISA. The Company and the ERISA Affiliates do not contribute to any plans which are subject to Title IV of ERISA. To the Company's knowledge, neither the Company nor any of the Subsidiaries has engaged in any non-exempt "prohibited transactions" under ERISA or the Code that could reasonably be expected to result in a Material Adverse Effect. The term " ERISA Affiliate " means each trade or business (whether or not incorporated) that would be treated as a single employer with the Company under Title IV of ERISA or Section 412 of the Code.

        (v)    The Company is not a party in interest under ERISA or a disqualified person under the Code to any employee benefit plan subject to the fiduciary responsibility part of ERISA or Section 4975 of the Code except for those employee benefit plans covering its employees or the employees of the Subsidiaries.

        (w)    Each of LaBranche & Co. LLC (" LaBCo. "), LaBranche Financial Services, Inc. (" LFS "), LaBranche Structured Products, LLC (" LSP ") and LaBranche Structured Products Specialists, LLC (" LSPS ") is (a) registered as a broker-dealer with the Commission under the Exchange Act and (b) registered with the securities authority of each state in which it is required to be so registered. Each of LaBCo., LFS and LSP is a member organization in good standing with the NYSE and is a member organization of the AMEX.

        (x)    Each of LaBCo., LFS, LSP and LSPS is a broker-dealer subject to the provisions of Regulation T (12 C.F.R. §220) of the Board of Governors of the Federal Reserve System. Each of LaBCo., LFS, LSP and LSPS maintains procedures and internal controls reasonably designed to ensure that it does not extend or maintain credit to or for its customers other than in accordance with the provisions of Regulation T, and management officials of the Company or the Subsidiaries regularly supervise the activities of LaBCo., LFS, LSP and LSPS and the activities of their respective employees in a manner reasonably designed to ensure that none of LaBCo., LFS, LSP or LSPS will extend or maintain credit to or for its customers other than in accordance with the provisions of said Regulation T.

        (y)    There is no franchise, contract or other document of a character that would be required to be described in a prospectus under the Securities Act, which is not described in or incorporated by reference in the Offering Document; and the statements set forth in the Offering Document under the headings "Risk Factors—Risks Related to Our Business—The regulatory environment in which we operate may change, making it difficult for us to maintain our levels of profitability," "—We are subject to extensive regulation under federal and state laws that could result in fines and other penalties" and "—We are subject to securities laws liability and related civil litigation," "Business—Our Specialist Segment—NYSE and AMEX Rules Governing Our Specialist Activities," "Business—Regulatory Matters," "Business—Legal Proceedings,' "Description of Senior Notes," "Description of Other Indebtedness," "Certain Federal Income Tax Consequences" and "Plan of Distribution," in the proxy statement filed April 12, 2004 under the heading "Employment Agreements" and in the 2003 Form 10-K under the headings "Business—Our Specialist Segment—NYSE and AMEX Rules Governing Our Specialist Activities," "Business—Regulatory Matters," "Business—Risk Factors—The regulatory environment in which we operate

6

may change, making it difficult for us to maintain our levels of profitability," and "—We are subject to securities laws liability and related civil litigation," "—Our or our employees' failure to comply with applicable laws and regulations could result (and have resulted) in substantial fines and other penalties," and "Legal Proceedings" fairly summarize in all material respects the matters described therein as of the date hereof and as of the date such statements were made.

        (z)    The Company and each of the Subsidiaries are insured by insurers of recognized financial responsibility against such losses and risks and in such amounts as are prudent and customary in the businesses in which they are engaged; all policies of insurance and fidelity or surety bonds insuring the Company or any of the Subsidiaries or any of its or their businesses, assets, employees, officers and directors are in full force and effect; the Company and the Subsidiaries are in compliance with the terms of such policies and instruments in all material respects; and there are no claims by the Company or any of the Subsidiaries under any such policy or instrument as to which any insurance company is denying liability or defending under a reservation of rights clause; neither the Company nor any of the Subsidiaries has been refused any insurance coverage sought or applied for; and neither the Company nor any of the Subsidiaries has any reason to believe that it will not be able to renew its existing insurance coverage as and when such coverage expires or to obtain similar coverage from similar insurers as may be necessary to continue its business at a cost that would not have a Material Adverse Effect, whether or not arising from transactions in the ordinary course of business, except as set forth in or contemplated in the Offering Document.

        (aa)    Other than the Registration Rights Agreement, there are no contracts, agreements or understandings between the Company and any person granting such person the right to require the Company to include any securities of the Company with Offered Securities registered pursuant to any Exchange Offer Registration Statement.

        (bb)    The Company is subject to the reporting requirements of either Section 13 or Section 15(d) of the Exchange Act and files reports with the Commission on the Electronic Data Gathering, Analysis, and Retrieval (EDGAR) system.

        (cc)    Neither the Company nor any of the Subsidiaries or any agent thereof acting on its or their behalf has taken, and none of them will take,